For a better understanding of the present invention, a ranging process of an OFDMA scheme will now be described in brief.
An IEEE (Institute of Electrical and Electronics Engineers) 802.16 communication system uses an orthogonal frequency division multiplexing/orthogonal frequency division multiple access (OFDM/OFDMA) scheme. A mobile station requires ranging channels and ranging codes in a ranging process. A base station allocates ranging codes, which are usable according to ranging purposes, that is, ranging types, to the mobile station.
For example, in an initial stage in which the mobile station enters a base station network, the base station allocates ranging codes related to initial ranging to the mobile station. While the mobile station performs handover, the base station allocates handover ranging codes related to handover to the mobile station. If the mobile station periodically implements ranging after entering the network, the base station allocates periodic ranging codes used for periodic ranging. If the mobile station requests a bandwidth for data communication, the base station allocates bandwidth request ranging codes to the mobile station. That is, the base station allocates ranging channels and ranging codes corresponding to ranging purposes to the mobile station, by request of mobile stations.
Ranging types classified according to ranging purposes, for example, initial ranging, handover ranging, bandwidth request ranging, and periodic ranging, will now be described.
The initial ranging is for the mobile station to adjust synchronization with the base station. The mobile station and the base station adjust accurate time offset and accurate frequency offset and control transmission power, etc by performing the initial ranging.
The periodic ranging is periodically conducted by the mobile station, which has performed the initial ranging with the base station, to adjust a channel state, etc. with the base station. To this end, the mobile station uses ranging codes allocated for the periodic ranging.
The bandwidth request ranging is for the mobile station, which has performed the initial ranging with the base station, to request that the base station allocate a bandwidth for actual communication with the base station.
FIG. 1 is a diagram illustrating a frame structure of a broadband wireless access system using an OFDMA scheme.
The OFDMA scheme is a two-dimensional access method which is a combination of a time division access (TDA) technique and a frequency division access (FDA) technique. The OFDMA scheme is a wireless access method for simultaneously transmitting information to a plurality of mobile stations by allocating sub-carriers having different orthogonality to a plurality of users. In transmitting data using the OFDMA scheme, respective OFDMA symbols are carried on subcarriers and are then transmitted through subchannels. The subchannels refer to channels comprised of a plurality of subcarriers. In a communication system using the OFDMA scheme, a predetermined number of subcarriers may constitute one subchannel according to system environments.
Referring to FIG. 1, a horizontal axis represents an OFDMA symbol number, and a vertical axis represents a subchannel number. A frame refers to a data sequence channel transmitted for a prescribed time period by physical characteristics and includes a downlink subframe and an uplink subframe. A preamble, which is a constituent of the frame, refers to specific sequence data located at the first symbol of every frame and is used for a mobile station to synchronize with a base station or to estimate a channel.
In the frame structure, a frame control header (FCH) provides information as to channel allocation and channel codes related to a downlink map (DL-MAP). The DL-MAP indicates a medium access control (MAC) message informing a mobile station of channel resource allocation in a downlink. An uplink map (UL-MAP) is a MAC message informing the mobile station of channel resource allocation in an uplink. A burst refers to a data unit transmitted to or received from one mobile station. The size and location of the burst are identified through the DL-MAP and the UP-MAP.
A ranging subchannel will now be described with reference to FIG. 1. A random access channel can be a ranging channel or a bandwidth request channel according to usage. The ranging channel may mean an initial ranging channel, a handover ranging channel, or a periodic ranging channel according to usage.
A code division multiple access (CDMA) scheme may be used in a random access process. The ranging channel is divided into resource units capable of transmitting a CDMA code and is called ranging request opportunity or bandwidth request opportunity. The number of OFDMA symbols assigned to the CDMA code differs according to usage. For example, two OFDMA symbols may be used for initial ranging, and one or three OFDMA symbols may be used for bandwidth request ranging according to base station setting.